The default notification tone can vary, dependent upon which variant of the ONE you have.
Do you have an unlocked variant or, a Carrier specific model?
Sorry,I missed the fact that you're on AT&T,& I'm assuming that you have their variant of the ONE.
In absence of a definitive answer,here's AT&T's support page for the ONE,w/links to Chat Line/User Forums,etc..
AT&T Wireless Support - Wireless Phone Device Tutorials & Account Support